Method and apparatus for squelch gating a receiving signal

Method and apparatus for squelch gating a receiving signal

The invention relates to an apparatus and a method for squelch gating a receiving signal.

In digital receivers the possibility to mute an audio output signal in case of disappearance of the carrier of the received audio signal or in case of a drop of the signal-noise-ratio in the received audio signal below a threshold is known from US 6,047,170 A.

The technical realisation for determining the measure of noise in a received signal is called squelch and is based on the continuous measuring of phase noise in the audio signal and the gating of the demodulated audio signal according the measured level of phase noise in comparison with specified thresholds.

Fig. 1 shows an apparatus for gating a received audio signal on the basis of a squelch signal characterising the measure of phase noise in the audio signal.

The received sampled RF audio signal x_n with its sampled cartesian components - sampled real component Re{x״} and sampled imaginary component Im{x״} - is converted in a Cordic-unit 1 to its polar components, sampled amplitude m_n and sampled phase φ_η . In a differentiating unit 3 of a squelch detector 3 a sampled phase difference Δφ_η between a sampled phase φ_η and its succeeding sampled phase φ_η__λ determined in a delay unit 4 of the differentiating unit 3 on the basis of the sampled phase φ_η - is determined in subtracting unit 5 of the differentiating unit 3.

The sampled phase difference Δφ_π contains signal portions of low frequency, caused by frequency offset or frequency drift of the carrier in the received RF audio signal, and phase noise as signal portions of high frequency. To eliG:\CPl2K\Letter Module\LettenByUsers\P\2006\P30489\2006l227J6-57_UU Unterlagen, usprtnglich_P30489_EP c minate the signal portions of low frequency in the sampled phase difference Δφ_η a subsequent highpass filter 6 of the squelch detector 2 - typically a 4^th order HR highpass filter - produces sampled phase differences Δφ_η comprising only phase noise relevant signal portions.

The signed sampled phase differences Δφ_η corresponding to phase noise are rectified in a subsequent squaring unit 7. The squaring unit 7 can alternatively be replaced by a device performing the absolute value of its input signal. For averaging the sampled unsigned RF phase differences p_nover time a digital filter 8 having a proportionalintegral characteristic concerning equation (1) represents the last signal processing unit of the squelch detector 2 delivering the sampled squelch signal s_n at its output. In equation (1) f_s represents the sampling rate and τ represents the time constant of the integral characteristic of the digital filter 8.

T

The sampled amplitudes m_n of the RF receiving audio signal x are demodulated in a demodulator 9. The demodulated

ח audio signal y_n is gated in a subsequent gating unit 10 on the basis of the determined sampled squelch signal s_n and a specified threshold delivering a muted audio signal a_nto the outputting unit 11.

In an off->on-»off-cycle of an audio transmission Fig. 2 displays the DC component of the carrier DC(m_n) (curve 40), the rectified RF phase noise p_n (curve 20) and the squelch signal s_n (curve 30) in the apparatus for gating a received audio signal according to Fig. 1, whereby the time constant τ of the integrating digital filter 8 ms is 5 ms.

The time constant of the digital filter is typically between 5 ms and 10 ms. Thus random peaks in the RF phase noise p״ being shorter than 5 ms to 10 ms are not averaged

G:\CPI2K\Letter Module\LettersBy(Jsers\P'2006\P30489\2006l227J6-57_UU Unlerlagen, usprunglich_P30489_EP.c by means of the digital filter 8 with proportional-digital characteristic leading to a squelch signal s_n with random peaks. Such peaks in the squelch signal s_n drops below respectively rises above the specified threshold(s) and 5 disadvantageously mutes the received audio signal for a short period. Especially in case of a squelch signal s_nrising above respectively dropping below a specified threshold leading to switch off respectively switch on of the received audio signal random peaks in the squelch 10 signal s_n occurring shortly after the switch reverses the switch of the received audio signal.

Prolongation of the time constant τ of the integrating digital filter 8 for avoiding such unwanted reversals of . 15 switches in the audio signal in case of peaks in the squelch signal would deteriorate the time behaviour of the integrating digital filter and thus the time behaviour of the squelch detector. The worse reaction of the integrating digital filter would unwontedly delay the 20 squelch signal s_n in comparison with an on-»off-step or an off-»on-step of the carrier in the audio transmission (see in Fig. 2 the delay of the squelch signal s_n (curve 20) in comparison with an off—>on -step of the DC component of the carpier DC(m_n) (curve 40) ) .

The object of the invention is to develop an apparatus and a method for squelch gating a received audio signal and whereby avoiding the occurrence of short-time reversal of the audio signal's switching without deteriorating the 30 time behaviour of the squelch detector.

The object is solved by a method with the features of claim 1 and an apparatus with the features of claim 8. The dependent claims comprise further developments of the 35 invention.

Inventively the start point resp. the end point of gating the audio signal is the time point, when the squelch signal s_n rises above the higher threshold delayed by a first

G:\CPI2K\Letter Module\LellasBy(Jsers\P\2006\P30489\2006l227_l6-57_UU Unierla8en, usprtjnglich_P3O489_EP.c delay time resp. drops below the lower threshold delayed by a second delay time. Therefore short-time reversal of ft switching the audio signal in case of occurrence of peaks in the squelch signal shortly after switching the audio signal can be advantageously prohibited.

In a first embodiment of the inventive method a third delay time for delaying the sampled amplitudes m_n of the audio signal at the start point of gating and a fourth 10 delay time for delaying the sampled amplitudes m_n of the audio signal at the end point of gating is calculated to compensate the unwanted delay in the muting of the demo$ dulated audio signal y_n caused by the first delay time at the start point of gating resp. by the second delay time at the end point of gating.

A delaying of the sampled amplitudes m_n of the audio signal by different delay times - third delay time at the beginning of gating, fourth delay time at the end of gating - can not be realised. Thus the maximum delay time of the third and fourth delay time is chosen for delaying the sampled amplitudes m_n of the audio signal both at the start and at the end of gating. The selection of one of the third or fourth delay time for delaying the sampled ך amplitudes m_n of the audio signal leads to a failure in the correct time point either for starting or for ending the gating. To eliminate this failure at one of the two time points both the first and the second delay time are recalculated by the chosen maximum delay time for delaying 30 the sampled amplitudes m_n of the audio signal.

For avoiding the delay between the squelch signal s_n and the demodulated audio signal y_n caused by the time constant r of the integrating digital filter 8 the user of 35 the receiver or an automatic selection unit can select a first time interval Δ/ relative to the original start point of gating - the time point, when the squelch signal s_n rises above the higher threshold delayed by the first delay time - for determining a new start point of gating

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and a second time interval Δζ״ relative to the original end point of gating - the time point, when the squelch signal s_n drops below the lower threshold delayed by the second delay time - for determining a new end point of 5 gating. The new start point of gating and the new end point of gating is determined empirically by the user or the automatic selection unit changing the first time interval Δί resp. the second time interval &t_ep, until phase noise disappears at the beginning resp. at the end 10 of the audio receiving.

In a second embodiment of the inventive method a third delay time for delaying the sampled amplitudes m_n of the audio signal at the start point of gating is calculated as 15 the difference of the original start point of gating and the new start point of gating being the summation of the first delay time and the first time interval Δί_χρ relative to the original start point of gating. Consequently a fourth delay time for delaying the sampled amplitudes m_n20 of the audio signal at the end point of gating is calculated as the difference of the original end point of gating and the new end point of gating being the summation of the second delay time and the second time interval Δζ_ρrelative to the original end point of gating.

To eliminate the failure caused by delaying the sampled amplitudes m_n of the audio signal both at the beginning and also at the end of gating both the first and the second delay time are recalculated each by subtracting the 30 first time interval Δί_χρ resp. the second time interval Δί_ΰρfrom the chosen maximum delay time for delaying the sampled amplitudes m_n of the audio signal.

For avoiding unwanted clicks at the beginning and/or at 35 the end of audio transmission the gating is continuously increased from an inactive gating level up to an active gating level during a first fading time T_/(M at the beginning of gating and is continuously decreased from an active gating level down to an inactive gating level

G:\CPl2K\Letter Module\LettersByUsers\P\2006\P30489\20061227_1657־_UU Unterlagen, usprijnglich_P3O489_EP.c during a second fading time Tj-_ade2 at the end of gating. In a third embodiment of the inventive method the start point of gating is the start point of fading from the inactive level and the end point of gating is the start point of fading from the active gating level. In a fourth embodiment of the inventive method the start point of gating is the end point of fading at the active gating level and the end point of gating is the end point of fading at the inactive gating level.

Optionally for reducing the delay between the squelch signal s_n and an on->off-step or an off->on-step of the carrier signal in the audio transmission the value of said squelch signal s_n is changed to the value of the higher threshold in case of a rising squelch signal s_n being below the higher threshold and the value of said squelch signal s_n is changed to the value of the lower threshold in case of a dropping squelch signal s_n being above the lower threshold.

An embodiment of the inventive apparatus and the four embodiments of the inventive method for gating a receiving signal will be described in detail in relation to the drawing. The figures of the drawing show:

Fig. 1 a block diagram of an apparatus for gating a receiving signal,

Fig. 2 a time diagram of RF phase noise, of squelch signal and of DC component of the carrier in an apparatus for gating a receiving signal according to Fig. 1,

Fig. 3 a block diagram of an inventive apparatus for gating a receiving signal,

Fig. 4 a flowchart of an inventive method for gating a receiving signal,

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Fig. 5 a time diagram of starting a gating of a receiving signal in a first embodiment of the invention,

Fig. 6 a time diagram of starting a gating of a receiving signal in a second embodiment of the invention,

Fig. 7 a time diagram of starting a gating of a receiving signal in a third embodiment of the invention,

Fig. 8 a time diagram of starting a gating of a receiving signal in a fourth embodiment of the invention,

Fig. 9 a time diagram of ending a gating of a receiving signal in a first embodiment of the invention,

Fig. 10 a time diagram of ending a gating of a receiving signal in a second embodiment of the invention,

Fig. 11 a time diagram of ending a gating of a receiving signal in a third embodiment of the invention,

Fig. 12 a time diagram of ending a gating of a receiving signal in a fourth embodiment of the invention and

Fig. 13 a time diagram of RF phase noise, of squelch signal and of DC component of the carrier in an inventive apparatus for gating a receiving signal.

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Firstly the inventive apparatus and the inventive method for gating a receiving signal are described on the basis of Fig. 3 and 4 of the drawing:

In the first step S10 of the inventive method for gating a receiving signal the sampled amplitudes m_n and sampled phase φ_η of the sampled receiving audio signal x_n are determined in an converter 1 - typically a Cordic-unit - on the basis of the sampled real component Re{x״} and the sampled imaginary component Im{x_n} of the complex receiving audio signal x_n .

In the following step S20 of the inventive method sampled rectified phase noise p_n is determined on the basis of the sample phases φ_η. Firstly the phase difference Δφ_η between a sampled phase φ_η and its succeeding sampled phase φ_η__χ is determined for each sampling point in a differentiating unit 3. Secondly the low frequency signal portions of the phase difference - caused by frequency offset and frequency drift of the carrier signal - are eliminated in a 4^th order highpass filter 6 to obtain a phase difference Δφ_η comprising only high frequency signal portions being phase noise p_n. In a squaring unit 7 the phase noise p_n is rectified. Alternatively the rectifying of the phase noise p can be performed by means of a absolute value performing unit.

In the next step S30 of the inventive method the squelch signal s_n is determined by averaging the rectified phase noise p_n in a digital filter 9 with proportional-integral characteristic.

Optionally in case of a rising squelch signal s_n being below a selected higher threshold S_hjgh the value of the squelch signal s_n is changed to the higher threshold S_highaccording to Fig. 13 in a gating unit 10. Consequently in case of a dropping squelch signal s_n being higher a selected lower threshold S_lnw the value of the squelch signal s_nis changed to the lower threshold S_low according to Fig. 13

G:\CP12K\Letter Module\LettersByUsers\P\2006\P30489\20061227_l657־_UU Unterlagen, uspriinglich_P30489_EP.c in the gate unit 10. By these means the time for starting resp. for ending the gating is thus reduced.

The following step S40 of the inventive method comprises the determination of the start point and of the end point of gating.

In a gating unit 10 the squelch signal s_n is compared with a selected higher threshold S_high in case of a rising of the squelch signal s_n. The start point SP_CA of gating in a first embodiment of the invention occurs according to Fig. 5, when the squelch signal s_n rises above the higher threshold S_high and a subsequent first delay time T_D]expires. The starting of gating is characterised by the rising audio gate signal AG - the control signal for gating the amplitudes m_n of the received audio signal in the gating unit 10 - in Fig. 5.

In a second embodiment of the invention the user of the receiver determines the start point SP_G2 of gating by adjusting the start point SP_GX of gating of the first embodiment of invention, until phase noise disappears at the beginning of gating. The start point SP_G2 of the second embodiment of the invention corresponds to an adjustment of the start point SP_G} of gating of the first embodiment of invention in the size of a first time interval Δζ according to Fig. 6. The adjustment in the size of a first time interval Δζ_ρ can be performed in both directions relative to the start point SP_Gl of gating of the first embodiment of invention.

In a third embodiment of the invention the gating is started from zero up to an active gating level inside a first fading time T_fodel . Defining the start of fading as the start point of gating the start point SP_G3 of the third embodiment of the invention corresponds to the start point SP_G2 of gating of the second embodiment of invention according to Fig. 7.

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In a fourth embodiment of the invention the end of fading defines the start point of gating leading to a start point

SP_(;4 of the fourth embodiment of the invention according to Fig. 8.

For determining the end point of gating the dropping squelch signal s_n is compared with a selected lower threshold S_low in the gating unit 10. The end point EP_GX of gating of the first embodiment of the invention occurs according to Fig. 9, when the squelch signal s_n drops below the lower threshold S_low and a subsequent second delay time T_D2 expires. The ending of gating is characterised by the dropping audio gate signal AG in Fig. 9.

In the second embodiment of the invention the end point׳ EP_G2 of gating is determined by adjusting the end point EP_gx of gating of the first embodiment of the invention, until the phase noise disappears at the end of gating. The end point EP_G2 of the second embodiment of the invention corresponds to an adjustment of the end point EP_GX of gating of the first embodiment of invention in the size of a second time interval Et_ep according to Fig. 10.

In the third embodiment of the invention with fading from an active gating level down to zero inside a second fading time T_fade2 at the end of gating according to Fig. 11 the end point EP_G2 of gating corresponds to the end point EP_G2of the second embodiment of the invention, if the start of fading defines the end point of gating.

According to Fig. 12 the end point EP_Gi: of gating in the fourth embodiment of the invention corresponds to the end point EP_g2 of the second embodiment of the invention in addition to the second fading time T_fade2, if the end of fading defines the end point of gating.

In the following step S50 of the inventive method the third delay time T_Di for delaying the sampled amplitudes m_n of the received audio signal at the beginning of the

G:\CPI2K\Letler Moduie\LettersByUsers\P\2006\P30489\20061227_l6-57_UU Unterlagen, usprunglich_P30489_EP.c gating and the fourth delay time T_M for delaying the sampled amplitudes m_n of the received audio signal at the end of the gating are calculated.

The third delay time T_D3 at the beginning of the gating in the first embodiment of the invention calculates according to equation (2a):

(2a)

The third delay time T_D3 at the beginning of the gating in the second embodiment of the invention calculates according to equation (2b), whereby a positive value of the first time interval Δί corresponds to a start point SP_G2of gating of the second embodiment of invention being earlier than the rising of the squelch signal s_n above the higher threshold S_high:

T_l)3>T_m+Δί_χρ (2b)

The third delay time T_D3 at the beginning of the gating in the third embodiment of the invention calculates according to equation (2c), whereby a positive value of the first time interval Δζ_ρ corresponds to a start point SP_G3 of gating of the third embodiment of invention being earlier than the rising of the squelch signal s_n above the higher threshold S_high :

^03 - ⁺ Δϊ.φ (2 c)

The third delay time T_D3 at the beginning of the gating in the fourth embodiment of the invention calculates according to equation (2d), whereby a positive value of the first time interval Δζ_ρ corresponds to a start point SP_C4of gating of the fourth embodiment of invention being earlier than the rising of the squelch signal s_n above the higher threshold S_high :

Td3 - Ρβ! ⁺ ^sp ~ I'fadel (2d)

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The fourth delay time T_Di at the end of the gating in the first embodiment of the invention calculates according to equation (3a):

T >T ^J W4 - ¹ D2 (3a)

The fourth delay time T_D4 at the end of the gating in the second embodiment of the invention calculates according to equation (3b), whereby a positive value of the second time interval Et corresponds to an end point EP_G2 of gating of the second embodiment of invention being earlier than the dropping of the squelch signal s_n below the lower threshold S_low׳.

^>4 - ^D2 ⁺ kt_ep (3b)

The fourth delay time T_D4 at the end of the gating in the third embodiment of the invention calculates according to equation (3c), whereby a positive value of the second time interval Δζ_ρ corresponds to an end point EP_G3 of gating of the third embodiment of invention being earlier than the dropping of the squelch signal s_n below the lower threshold ^:

7._μ>7._0!+Δ<3) ״c)

The fourth delay time T_D4 at the end of the gating in the fourth embodiment of the invention calculates according to equation (3d), whereby a positive value of the second time interval Δ/ corresponds to an end point EP_G4 of gating of the fourth embodiment of invention being earlier than the dropping of the squelch signal s_n below the lower threshold S_low:

^04 - ^02 ⁺ At_ep + Tf_ade₂ (3d)

For delaying the sampled amplitudes m_n of the received audio signal both at the beginning and also at the end of

G:\CPl2K\Letter Module\LettcrsByUsers\P\2006\P30489\2006l227_l6-57_UU Unterlagen, uspriinglich_P30489_EP.c the gating the maximum delay time T_DMax is chosen of the third delay time Τ_υ3 at the beginning of the gating and of the fourth delay time T_M at the end of the gating in the next step S60 according to equation (4):

T_DMax=^{W_D»T_D4} (4)

In step S60 the sampled amplitudes m_n of the received audio signal are delayed in a delay unit 12 located between the demodulator 9 and the gating unit 10 in the inventive apparatus for gating a received signal according to Fig. 3 with the maximum delay time T_DMax.

The failure caused by choosing one delay time of the third delay time T_D3 or the fourth delay time T_D4 for the delay time T_DMax applied for delaying the sampled amplitudes m_nof the received audio signal both at the beginning and at the end of the gating is compensated in the next step S70 by recalculating the first delay time T_DX and the second delay time T_D2 :

For the first embodiment of the invention the first delay T_D] is recalculated according to equation (5a) and the second delay time T_D2 is recalculated according to equation (6a):

I'd] ⁼ ?DMax (5a)

I'd2⁼I'dmox (6a)

For the second embodiment of the invention the first delay T_DX is recalculated according to equation (5b) and the second delay time T_D2 is recalculated according to equation (6b):

77, = T_I)M״ - ΔΓ״

DI DM ax sp

I'd! ~ Idmox At_ep (5b) (6b)

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For the third embodiment of the invention the first delay T_D\ is recalculated according to equation (5c) and the second delay time T_D2 is recalculated according to equation (6c):

⁼ ~ At,ip ( 5 C ) ^D1 ~ T-DMax. ~ At_ep ( 60 )

For the fourth embodiment of the invention the first delay T_D} is recalculated according to equation (5d) and the second delay time Τ_ϋ2 is recalculated according to equa¾., tion (6d):

= T_DM!a — Et_xp -i-1 j-_M (5d) ?!)2 ⁼ ΤβΜαχ ~ _ep ־־־ Tj-_ade2 ( 6 d)

By using the recalculated first delay time according 20 to one of the equations (5a) to (5d) and the second delay time T_D2 according to one of the equations (6a) to (6d) the new start point SP_G3, SP_G2, SP_G3 or SP_Gi of the first, second, third or fourth embodiment of the invention according to Fig. 5, 6, 7 or 8 and the end point EP_G}, 25 EP_G2,EP_G3 or EP_GA of the first, second, third or fourth

Τ' embodiment of the invention according to Fig. 9, 10, 11 or is determined in the next step S80.

In the last step S90 of the inventive method the delayed 30 sampled amplitudes m_n of the received audio signal are gated in the gating unit 10 between the start point

SP_CX , SP_G2, SP_G3 or SP_C4 in the first, second, third or fourth embodiment of the invention and the end point EP_G{, EP_G2, EP_G3 or EP_ga of the first, second, third or fourth embo35 diment of the invention.

The invention is not limited to the embodiment of the inventive apparatus and to the four embodiments of the inventive method for gating a receiving signal. The invenG:\CPI2K\Letter Module\LcttersByUsers\P\2006\P30489\2006l227_l6-57_UU Unterlagen, usprtjnglich P30489_EP.c tion also covers other combinations of the embodiments of the inventive method, for example a level and time trigger of the squelch signal s_n without using any user specific adjustment of the start point and/or end point of gating in combination with fading of the gating. Furthermore, nonlinear characteristic of fading - for example a squared fading characteristic - is also inside the scope of the invention.